Acid mine drainages (AMD) naturally occur at mine sites and usually contain acidity, sulphate and several heavy metals such as iron, zinc, copper, lead, manganese, aluminum, cadmium, nickel at varying degrees. The AMD, like other industrial wastewaters, must be treated for neutralization of acidity and removal of heavy metals prior to its release to the environment. One method of treating AMD is to use a neutralizing material. Although the solubility of the various heavy metals varies with the pH of the solution in which they are dissolved, most of the heavy metals can be kept insoluble at basic pH levels, i.e. 9-11 by adjusting the pH of the acidic wastewater with a neutralizing reagent. Lime is often recommended for neutralization because of its calcium ion content, simplicity and relatively low cost. Calcium ions form insoluble calcium salts such as calcium sulphate at neutral or alkaline pH levels whereas heavy metals are precipitated as their hydroxides. In a specifically designed process, calcium sulphate precipitates formed can play an environmentally safe binder role as nuclei for heavy metals precipitated by structuring the formation of stable crystals or crystalline particles. Metal hydroxide and calcium sulphate precipitates, commonly called "sludge", undergo a solid/liquid separation process. A clarifier/thickener, in which sludge settles by gravity, is a common device used for producing thickened sludge for disposal. Denser sludge composed of crystalline precipitates, settles better and faster resulting in an enhanced solid/liquid separation process and improved effluent quality. Due to denser sludge produced, the volume of the clarifier required can marginally be reduced and some savings can be obtained in the unit operation cost of the process. Reduction in sludge volume is also desirable in order to decrease the cost of sludge management (e.g. disposal and storage costs). Metal hydroxide sludges are usually not chemically stable; they are susceptible to changes in the environmental conditions such as pH. Metals in an unstable sludge are easily redissolved and are leached out from the sludge and report to the environment. Therefore, the sludge generated must be chemically stable.
The conventional lime neutralization process advocates simply the addition of lime as slurry to adjust the pH of the AMD to a desired level whereby heavy metals will be precipitated. Specified amount of air is also introduced to the water to oxidize ferrous iron to ferric iron for complete precipitation. The settled precipitates are gelatinous-like with low solids content, generally between 0.5-1 weight percent solids. Separation of gelatinous-like solids from the treated water is difficult and requires large expensive thickeners. In such a system, the precipitated calcium salts are not only removed with the precipitated heavy metals, but are also deposited on the surfaces of the equipment and piping used to treat or transport the wastewater, which is called scale formation. Layers of the salt accumulate and eventually clog the equipment causing periodic shut down of the equipment for removing deposits. Such maintenance increases the cost of the treatment process.
To prevent scale formation, lime slurry is first mixed with immense amount of polymer and is then used to neutralize AMD containing high sulphate levels. However, the pH must be adjusted to the desired level at various stages and residence times. Such a multistep or multistage process is time consuming and requires extra reactors for neutralization. The process is not effective for water containing less than 3600 mg/L sulphate. As a result, high sulphate requirement also limits the process from broad application. The solid content of the settled precipitates is not more than 10 percent, even, after treatment of high sulphate AMD.
Another process proposes the use of limestone as a neutralizing reagent to obtain denser sludges. Due to high buffering capacity of limestone at about pH 6-7, the pH of the acidic water cannot be raised to pH 9-10, which is necessary to precipitate a wide range of heavy metals present in AMD, with limestone alone. Limestone is effective in removing ferric iron. Ferrous iron must be oxidized to ferric iron prior to treatment. Oxidation of ferrous iron at acidic pH levels with air is almost impossible because of very slow reaction rates and requires expensive reagents and techniques such as use of hydrogen peroxide. The limestone should also have certain properties, such as high quality and very fine particles. To circumvent drawbacks of limestone neutralization process, a two-stage process in which limestone is first used to increase the pH to 6-7 and then lime is added to the water to obtain the desired pH, has been suggested. However, the two-stage method does not address problems associated with the oxidation of iron, generation of high density sludge and elimination of scaling.
In other methods, sludge with relatively high density and lower volume is generated. The methods are based on recycling a specified amount of sludge with a specified amount of solids to the process. It has been found that when the recycled sludge is used as a carrier for the neutralizing agent, a sludge with high solids and low volume is obtained. The neutralizing agent is adsorbed on the recycled sludge and that mixture is introduced to the acidic water to raise the pH to 8-9 in one step. The method is quite efficient in treating waters containing high levels of iron and small amounts of other heavy metals. However, the ratio of ferrous to ferric iron must be kept at a specific proportion, which is difficult and requires a well-controlled oxidation process, to be able to obtain expected results. Removal of a wide range of heavy metals cannot be achieved by adjusting the pH to 8-9 in one step, since removal of some metals (e.g. cadmium, lead) requires higher pH levels (i.e. pH 10-11) where a portion of metals precipitated at lower pH levels will be dissolved. In addition to production of poor final effluent quality, the process cannot resolve the problems associated with the precipitation of insoluble calcium salts ("scaling").